Why the summer guys? It's hot and shitty. That, and us office types don't get vacations anymore. In December, it's nice and cool, the schoolkids have Christmas vacation, and everyone gets a Christmas bonus in one form or another. Perfect for buying all sorts of crap.
Most people come out and stay in malls around summer. They like malls because it's breezing while in the outside it's hot. I choose April-May, the summer. Well of course, not everyone likes it but that's the time everyone should meet and sign.